Correct,the Wobbuffett does not take any damage and the benched poke`s will take the 20 damage.Ruling below.

`Sensei

Q. What happens if Aggron-EX uses "Metal Surge" VS. an active
Wobbuffet? Does Wobbuffet take no damage and all benched pokemon take 20
damage? Or does the "Safeguard" PokeBody prevent damage and effects done to
the benched pokemon too?